BACKGROUND: The prevalence of mental disorders among prisoners is considerably higher than in the general population. This is an important public health issue as the vast majority of prisoners stay in custody for less than 9 months and, when not in prison, offenders' lifestyles are frequently chaotic, characterized by social exclusion, instability and unemployment. Multi-disciplinary mental health inreach services were introduced to target care towards prisoners with severe mental illness (SMI) in a similar way to that provided by Community Mental Health Teams outside prison. The aim was to establish the proportion of prisoners with SMI who were assessed and managed by prison mental health inreach services. Method A two-phase prevalence survey in six prisons in England measured SMI upon reception into custody. Case-note review established the proportion of those with SMI subsequently assessed and treated by inreach services. RESULTS: Of 3492 prisoners screened, 23% had SMI. Inreach teams assessed only 25% of these unwell prisoners, and accepted just 13% onto their caseloads. CONCLUSIONS: Inreach teams identified and managed only a small proportion of prisoners with SMI. Prison-based services need to improve screening procedures and develop effective care pathways to ensure access to appropriate services. Improved identification of mental illness is needed in both the community and the Criminal Justice System to better engage with socially transient individuals who have chaotic lifestyles and complex needs.